The present invention relates to a complexing package used to enhance the rheology and green bonding properties of adhesive compositions, and which further provides improved water resistance. More particularly, the invention relates to starch-based adhesives whose green bonding and moisture resistance have been improved by the addition of a complexing package that includes a bivalent metal oxide.
Corrugated paperboard or liner board is commonly prepared by first forming a corrugated element or medium by passing a cellulosic sheet between corrugating rolls that form a substantially sinusoidal cross-section in the sheet, commonly referred to as the fluted layer. An adhesive is applied to the tips of the thus formed sinusoidal portion, or flutes. A non-corrugated or planar cellulosic liner, commonly referred to as a facing layer, is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roll. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced element. The single-faced element may be used in applications as a liner or buffer material within a container. More commonly, adhesive is applied to the flute tips of the single-faced element and a second liner sheet is subsequently applied to the adhesive liner in a xe2x80x9cdouble facerxe2x80x9d operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. Following application of the adhesive, the bond between the corrugating medium and the facing layer(s) is developed by passing the corrugated structure continuously through an oven at temperatures ranging from about 300xc2x0 to 400xc2x0 F., thus establishing a fully cured adhesive bond(s).
The adhesives which have been developed to form corrugated paperboard are generally starch-based adhesives, due to the desirable adhesive properties, low cost and ready availability of starch-based products. The most common starch corrugator adhesives in use today are alkaline in character. The earliest corrugated paperboard adhesive patent, U.S. Pat. No. 2,102, 937, discloses an adhesive composition comprising gelatinized starch, ungelatinized starch, alkali and borax in a water base. This adhesive formulation has formed the basis for the manufacture of essentially all corrugated paperboard to date.
In general, starch corrugating adhesives are produced by gelatinizing a relatively small portion of the starch in water with caustic soda to yield a primary mix of gelatinized carrier, which is then slowly added to a secondary mix of the remaining raw (ungelatinized) starch, borax, and water to produce the full-formulation adhesive. During the corrugating process heat is used to increase the viscosity of the adhesive that has been applied to the tips of the flute tips. The applied heat increases the viscosity of the adhesive by swelling the raw starch, or secondary component, through absorption of water from the carrier starch, or primary component. This swelling of the raw starch imparts the initial, or green, bonding (also referred to as tack) between the fluted layer and the facing layer.
The speed at which an adequate green bond is formed is an important aspect of the adhesive formulation. The green bond secures the fluted layer and the facing layer together prior to and during the full cure developed in the oven. Thus, higher production speeds are possible for starch formulations with higher green bond strength. Throughput increases are particularly important to paperboard manufacturers because corrugation is typically a high speed process. For example, a typical commercial bonding apparatus for continuously joining strips of paper to form corrugated paperboard is designed to operated at speed ranging between 150 and 1000 feet of paperboard per minute. Thus, the speed at which corrugating machines can be run is limited by the rapidity of the viscosity increase in the bond line between the fluted layer and the liner layer(s). Therefore, increases in the green bond strength of starch-based adhesives typically employed in paperboard manufacture are highly beneficial.
Moisture resistance is also considered highly beneficial in many corrugated paperboard applications, particularly resistance to high humidity, liquid water and the like. Conventional alkaline gelatin adhesives generally exhibit poor moisture resistance. Acidic starch formulations have been developed to address this problem; however, acidic starch-based adhesives generally have poor green bond strength due to higher gelation temperatures. Various additives have been proposed to improve the green bond strength of acid starch formulations; however the use of such additives is cost prohibitive. Further, such acidic starch compositions suffer from poor viscosity stability, short shelf life, and excessive formaldehyde odor. Acidic corrugating adhesives also tend to be corrosive. Moisture resistant alkaline starch formulations have likewise been proposed. However, the additives generally employed in such moisture resistant alkaline starch compositions significantly decrease the shelf life of the adhesive formulation. Further, the additives employed are commonly formaldehyde based, giving rise to environmental concerns.
Thus there remains in the art a need for starch-based adhesives having superior green bond strength, improved moisture resistance, and which are further cost effective to produce and have adequate shelf life.
The present invention provides a complexing package for alkaline starch-based adhesive formulations that imparts superior green bond strength and improved moisture resistance, is cost effective, does not pose the environmental concerns associated with formaldehyde, and possesses adequate shelf-life. The complexing package of the present invention thus permits conventional alkaline starch-based adhesives to be used at higher corrugator speeds while simultaneously providing a more moisture resistant product. It has further been found that inclusion of the complexing package of the present invention leads to starch formulations having more uniformly distributed starch particles that are generally more closely spaced, as well as providing a more consistent viscosity to the resulting adhesive. It is further believed that the complexing package of the present invention may find utility in a wide range of compositions in which such rheological modification would be beneficial, in addition to alkaline starch-based adhesives.
The present invention provides complexing packages that include at least one bivalent metal salt derived from a bivalent metal oxide and at least one polymeric vehicle, the polymeric vehicle capable of transporting the bivalent metal salt into the starch formulation. Exemplary bivalent metal oxides include zinc oxide, calcium oxide, magnesium oxide, lead oxide, copper oxide, aluminum oxide, iron oxide, titanium oxide, and mixtures thereof. In advantageous embodiments, the bivalent metal oxide is zinc oxide. Exemplary effective amounts of the bivalent metal oxide range from about 2 to about 4 wt % based on the weight of the complexing package (xe2x80x9cbocxe2x80x9d). Exemplary polymeric vehicles include alkali stable carboxylated polymers and polyvinyl alcohol. Advantageously, the polymeric vehicle comprises a carboxylated styrene-butadiene polymer. The polymeric vehicle may be present in the complexing package in effective amounts ranging from about 12 to about 20 wt % boc.
In further advantageous embodiments, starch-based adhesives are provided which include the complexing package of the present invention. The complexing package can be included in the starch-based adhesive in amounts ranging from about 1.3 to about 1.7 wt %, based on the weight of the adhesive. Adhesives incorporating the complexing package of the present invention have improved green bonding properties, believed to stem from improved theological properties. The improved rheological properties imparted by the complexing package of the present invention is further believed to translate into other highly beneficial properties, such as increased consistency in the viscosity of the adhesive. The invention further relates to corrugated paperboard incorporating the complexing package and starch-based adhesives of the present invention.